1. Field of the Invention
This invention relates to variable-magnification optical systems having the function of image stabilization and, more particularly, to a variable-magnification optical system of which part, or a lens unit, is made to relatively tilt with its tilting center at a point on an optical axis so that when the optical system vibrates (tilts), shaking of a photographed image is optically corrected to obtain a stationary image, thus achieving good stabilization of the photographed image, and getting the function of image stabilization suited to photographic cameras or video cameras.
2. Description of the Related Art
When a photograph is taken from a moving car, flying aircraft, or moving vehicle, vibrations propagate to the photographic system, causing a photographed image to shake.
There have been proposed a wide variety of optical systems having the function of preventing an image of an object being photographed from shaking, i.e., or image-stabilizing optical systems.
For example, Japanese Patent Publication No. Sho 56-21133 discloses an optical instrument, in which vibrations thereof are detected by a sensor, and, in response to an output signal of the sensor, an optical member constituting part of the optical instrument is moved in such a direction as to cancel a vibratory displacement of the image caused by the vibrations, so that the image is stabilized.
Japanese Laid-Open Patent Application No. Sho 61-223819 discloses a photographic system with a refraction type variable angle prism arranged in the frontmost position, wherein the apex angle of the prism is varied in response to vibrations of the photographic system to thereby deflect the image. Thus, the image is stabilized.
In Japanese Patent Publications Nos. Sho 56-34847 and Sho 57-7414, the photographic system is provided with a component or an optical member arranged to be spatially fixed against vibrations. As the photographic system vibrates, this optical member produces a prism effect by which the photographed image is deflected. Thus, a stationary image is obtained on the image plane.
Also, in Japanese Laid-Open Patent Application No. Sho 50-137555, for a telephoto lens, a lens unit on the object side is made to tilt around a point on an optical axis its focal length distant from its principal point, so that shaking of the photographed image is corrected when the telephoto lens vibrates.
In Japanese Laid-Open Patent Application No. Sho 63-115126, another method is employed by utilizing an acceleration sensor or the like in detecting vibrations of the photographic system. Responsive to the signal produced by the acceleration sensor, a lens unit constituting part of the photographic system is made to shift in a direction perpendicular to the optical axis, so that a stationary image is obtained.
In addition, Japanese Laid-Open Patent Application No. Hei 2-238429 and U.S. Pat. No. 2,959,088 propose the utilization of an inertial pendulum mechanism in the image stabilizing system. A lens system composed of two lens units or first and second lens units of negative and positive powers is arranged in front of the photographic system, and the second lens unit is made movable as the photographic system vibrates. For image stabilization, the second lens unit is supported in gimbal fashion on its focal position.
In general, the image-stabilizing instrument of the type using the method of tilting a movable lens unit of the image-stabilizing optical system placed in front of the photographic system to remove the shaking of the photographed image so as to obtain a stationary image has problems that the instrument as a whole gets a large size and that a moving mechanism for the movable lens unit becomes complicated in structure.
Another problem arises in that the tilting of the movable lens unit produces a large amount of decentering aberrations, causing the optical performance to be significantly reduced.
The image-stabilizing optical system utilizing the variable-angle prism has a problem that at the time of the image stabilizing operation, the amount of decentering lateral chromatic aberrations increases largely as zooming goes particularly to the positions of long focal lengths (telephoto side).
Meanwhile, the image-stabilizing optical system of the type in which a part of the photographic lens system is shifted in parallel in a direction perpendicular to the optical axis, though having an advantage that there is no need of a special optical system for image stabilization, has a problem that at the time of the image stabilizing operation, a large amount of decentering aberrations is produced.
Again, in order to assure a necessary amount of light on the image plane at the time of the image stabilizing operation, those of the lens units which lie on the object side of the movable lens unit are required to increase their diameters. For this reason, there is a problem that the whole instrument gets a large size.
Further, in the image-stabilizing optical system of the type in which the image stabilizing operation is effected by tilting at least one lens unit behind the variable magnification lens units (zoom section), the relationship between the amount of correction for the image shake and the amount of movement of the movable lens unit varies with variation of the zooming position in a complicated manner. To evaluate these amounts, a computing circuit or like means becomes necessary. So, there is a problem that the whole instrument gets an increased complexity of structure, and a large increase of the production cost results.
In another case where the variator or another lens unit movable for zooming is used in stabilizing the image, a problem arises in that an operating mechanism becomes very complicated, and the load on a driving means for those lens units, namely, a zoom motor, increases largely.
Further, in the proposal of Japanese Laid-Open Patent Application No. Hei 2-238429 and U.S. Pat. No. 2,959,088 mentioned before, use is made of a negative first lens unit and a positive second lens unit as arranged in front of the photographic system, wherein the second lens unit is supported by a gimbal. Such an image-stabilizing optical system employing the principle of inertial pendulum has a characteristic feature that even if the vibrations have a high frequency, the image stabilizing function works considerably better than with the before-described active type of image-stabilizing optical systems.
Because the lens units which are effective only for image stabilization are positioned in front of the photographic system, and because one of these lens units is made to oscillate so as to correct the image shake so that a stationary image is obtained, there is, however, a problem that the whole lens system is required to have a large size.
Meanwhile, in order to prevent the optical performance from decreasing at the time of the image stabilizing operation, part of the photographic system, or a lens member, may be shifted in parallel in a direction perpendicular to the optical axis. When large angles of correction are required in such an image-stabilizing optical system, it is very difficult to sufficiently correct the decentering aberrations at the time of the image stabilizing operation. Further, in an optical system using the variable angle prism, there is a problem that large decentering lateral chromatic aberration is produced at the time of the image stabilizing operation.